(1) Field of the Invention
The present invention relates to a transmission system, and more particularly to a transmission system for transmitting signals under flow control.
(2) Description of the Related Art
In recent years, information communication networks are constructed in various many forms, and are expected to have improved communication quality and service performance in view of growing transmission capacities. Under the circumstances, loads in networks are rapidly increasing, and the importance of a flow control process for adjusting a flow of data in networks is also increasing.
The flow control process is a control process for sending data over a network to a recipient in a manner to match the reception ability of the recipient or the traffic of the network. For example, if the data transfer rate of the sender is higher than the data processing rate of the receiver, then the data transfer between the sender and the receiver is confirmed based on flow control information and performed reliably so that the data received by the receiver will not overflow the buffer of the receiver and not cause a data loss.
According to a conventional flow control process, a flow control signal indicative of data reception (XON) or no data reception (XOFF) is sent and received for flow control (see, for example, Japanese Unexamined Patent Publication No. 10-13441 (paragraphs [0016]-[0059], FIG. 2)).
There is proposed a flow control technique for limiting an inflow of cells before a buffer is congested and cells are dropped, thereby preventing cells from being dropped and retransmitted (see, for example, Japanese Unexamined Patent Publication No. 7-66820 (paragraphs [0012]-[0056], FIG. 1)).
The conventional flow control process is disadvantageous in that if data with flow control information assigned thereto suffers an error due to a fault or the like on the transmission path, then since the sender is unable to determine the contents of the flow control information, the flow control information becomes invalid, and no flow control can be performed. Furthermore, if the system is caused to malfunction by the invalidation of flow control information, then the system cannot autonomously be recovered.
Flow control information can be assigned to data in two formats, a bitmap format and an event format. The bitmap format allows flow control information to be set up for all circuits handled by the system, whereas the event format serves to set up flow control information for one circuit.
Usually, the event format is used in many occasions in order to reduce the capacity of the transmission path. However, if data flow control information assigned thereto in the event format undergoes an error, then the sender is totally unable to recognize the congestion state of the circuit, i.e., whether the circuit is in a congested state or a recovered state free of a congestion.
The reasons for the above problem are as follows: According to the event format, only when an event such as the occurrence of a congestion or the removal of a congestion occurs in a certain single circuit, flow control information for the circuit is set up and transferred to the sender. If the sender is unable to receive the flow control information properly, then the sender will subsequently have no opportunity to receive the flow control information.
Consequently, the invalidation of flow control information on account of a data transmission error has a significant impact on the event format, and has been responsible for a reduction in the transmission quality and reliability in the conventional data transmission systems.
The former conventional flow control process (Japanese Unexamined Patent Publication No. 10-13441) is a flow control process specialized in Ethernet (registered trademark) applications. The latter conventional flow control process (Japanese Unexamined Patent Publication No. 7-66820) is a flow control process effective for networks having the ring topology. Accordingly, these flow control processes are not applicable as an event-format flow control process in a wide range of data transmission tasks. In addition, the above conventional flow control processes give no consideration to effective countermeasures against errors that occur during the transmission of flow control information.